Apoptosis is a process of programmed cell death mediated by a number of signaling pathways that converge at the mitochondria. A group of mitochondrial proteins, i.e., the Bcl-2 proteins, regulates this process. More specifically, pro-apoptotic and anti-apoptotic Bcl-2 proteins form heterodimers with their cognate regulating Bcl-2 proteins (i.e., the BH3-only Bcl-2 proteins), thereby executing cell death or survival signals.
Essentially all effective cancer drugs induce apoptosis in target cancer cells. However, different cancer cells respond to an apoptosis-inducing drug in different manners. This is due to the presence of different heterodimers between the pro/anti-apoptotic Bcl-2 proteins and the regulatory BH3-only Bcl-2 proteins in those cancer cells. Determining the presence of these heterodimers in a cancer patient helps assessing that patient's responsiveness to an apoptosis-inducing cancer drug.